The NTP Roadmap for the 21st Century includes a major initiative to develop HTS assays to: (1) identify mechanisms of action for further investigation, (2) develop predictive models for biological response, and (3) prioritize substances for further toxicological evaluation. In support of this initiative, NTP became a formal participant in the NIH Molecular Libraries Initiative (MLI). The MLI [unreadable] a part of the NIH Roadmap for Medical Research [unreadable] is focusing on the use of high-throughput technology screening methods to identify small molecules that can be optimized as chemical probes to study the functions of genes, cells, and biochemical pathways. The NTP, through its association with the MLI, has the opportunity to generate information that links data on the biological activity of substances generated from biochemical and cell-based HTS assays with toxicity endpoints identified in the NTP's toxicology testing program. To achieve this goal, the NTP initiated a formal collaboration with the NIH Chemical Genomics Center (NCGC) to test substances of interest to the NTP across a spectrum of HTS assays. The NCGC is the only HTS center in the world that screens all assays at multiple concentrations in the primary screen, a process called Quantitative High Throughput Screening (qHTS). This process is critical to meeting the goals of the NTP, since unlike traditional HTS performed at a single concentration, false negatives are minimized and a pharmacological activity profile of every compound is obtained. In this collaboration, NTP supplies compounds of toxicological interest and HTS assays are selected jointly, after careful consideration of the technology by the high throughput screening experts at NCGC and the biology by toxicologists at NTP. Through this collaboration batteries of HTS assays are being developed that will allow for substances which lack needed toxicological data to be prioritized for further in-depth toxicological evaluation, for mechanisms of action to be identified for further investigation, and for predictive models for in vivo biological response to be developed. Ultimately, the use of HTS assays will greatly increase testing throughput and decrease the cost of testing of potentially toxic compounds. This collaboration between the NTP and the NCGC has expanded to include the EPA National Center for Computational Toxicology (NCCT), forming a Tox21 consortium. The NCCT, through Tox21, has also proposed HTS assays for screening at NCGC and the NCCT has also provided several thousand compounds of toxicological interest to the NCGC for HTS testing. Both the NTP and the EPA compound libraries (jointly referred to as the Tox21 library), which should total 10,000 compounds by early 2010, may be tested in the HTS assays provided by NTP and EPA;NTP may screen its library separately on occasion, in special assays developed and submitted by NIEHS DIR scientists. The data resulting from joint screening will be shared among all three parties.